1. Field of the Invention
The present invention relates to a high voltage supply device, and more particularly, to a high voltage supply device for substantially eliminating a surge voltage occurring upon turning off electric power.
2. Description of the Related Art
A high voltage supply device is commonly used for electronic devices, such as laser printers and facsimile machines, requiring a high dc voltage source. Such a high voltage supply device may convert a dc voltage into a high ac voltage, and regulate and convert the ac voltage into a high dc voltage. The high voltage supply generally has a transformer for converting a dc voltage into an ac voltage.
The high voltage supply device may generate an undesired surge voltage while converting dc voltages into ac voltages when using a transformer. An electronic device having such a high voltage supply device may induce a high surge voltage when power is turned off, by discharging a dc voltage applied to the transformer, and such a surge voltage may partially or totally damage the electronic device. In the case of a laser printer, a surge voltage, occurring at the time electric power applied to the printer is cutoff, may induce another surge voltage to be applied to an organic photo conductor drum, which may damage the drum. In addition, electric power applied to the high voltage supply device while in the stand-by mode is unnecessarily consumed.
FIG. 1 is a block diagram schematically showing a conventional high voltage supply device. The high voltage supply device shown in FIG. 1 has a power supply 10, a controller 20, and a high voltage generator 30. The power supply 10 rectifies an ac voltage externally applied, and generates dc voltages of, for example, 24V and 5V. The voltage of 24V may be used as an operation voltage for the high voltage generator 30, and the voltage of 5V may be used as an operation voltage of the controller 20.
The controller 20 outputs a pulse width modulation (PWM) signal having a predetermined duty ratio according to a preset value.
The high voltage generator 30 generates ac voltages ranging from a few hundred volts to a few thousand volts by on and off switching operations by a PWM signal applied from the controller 20.
The high voltage generator 30 includes a switching part 31, a transformer 32, and a rectifier 33.
The switching part 31 is repeatedly turned on and off by a PWM signal applied from the controller 20. For example, when the switching part 31 is turned off by a PWM signal from the controller 20, the voltage 24V cuts off a current path on an input side 32a of the transformer 32.
The transformer 32 induces a high ac voltage across output terminals 32b from the ac voltage of 24V turned on and off by the switching part 31. The number of wire windings of the output terminals 32b may be greater than the number of wire windings of the input terminals 32a in order to output a voltage higher than an applied voltage.
The rectifier 33 rectifies, and converts to a dc voltage, an ac voltage outputted from the output terminals 32b of the transformer 32. The rectifier 33 uses a N-times multiple voltage rectification method to heighten a voltage outputted from the transformer 32.
At substantially the same time, the controller 20 sets the switching part 31 to be enabled in order to enhance the stability of signal inputs and outputs when an outputted PWM signal is a logic “low,” generally referred to as an active low; one of methods frequently used in digital logic. Accordingly, when the dc voltage of 5V applied to the controller 20 is cutoff, a PWM signal outputted from the controller 20 becomes the logic “low” and, at this time, the switching part 31 may malfunction. Further, since the operation voltage of 5V of the controller 20 is very low compared to the operation voltage of 24V of the high voltage generator 30, the voltage source of 24V has a potential level higher than the voltage source of 5V at the time the voltage source of 5V becomes a ground level when the high voltage supply device is turned off. For example, even when the operation voltage of the controller 20 of 5V is lowered to a value equal to, or less than, 2.5V, the dc voltage of 24V has a potential level of about 18V. Accordingly, when a PWM signal outputted from the controller 20 is a logic “low,” the switching part 31 is turned on so that the input terminals 32a of the transformer 32 form a current path between the voltage potential of 18V and the ground terminal. In this case, a high ac voltage is induced across the output terminals 32b of the transformer 32, and rectified and outputted by the rectifier 33.
FIG. 2 is a view showing an output voltage waveform of a high voltage supply device when the high voltage supply device of FIG. 1 is turned off.
As shown in FIG. 2, a dc voltage e.g., 18V exists across the input terminals 32a of the transformer 32 and the ground terminal at the time labeled 1, when an ac voltage applied external to the power supply 10 is cut off, so that the voltage of 5V is lowered to a value equal to, or less, than 2.5V. At this time, since the voltage of 5V outputted from the power supply 10 drops to a value substantially equal to, or less than, 2.5V, the power supply 10 does not drive the controller 20 which operates at 5V, and a PWM signal outputted from the controller 20 becomes the logic “low.” However, when the controller 20 is turned off at point 1, the dc voltage of 18V outputted from the power supply 10 has been applied to the input terminal of the transformer 32. Accordingly, when the potential level of the PWM signal outputted from the controller 20 becomes the logic “low,” a voltage, labeled “a”, exists between the input terminal 32a of the transformer 32 and the ground so that a high ac voltage e.g., surge voltage is induced across the output terminals 32b of the transformer 32.
As described above, a high dc voltage, applied to electronic devices such as laser printers, facsimile machines, or the like, may damage parts of electronic devices when the devices are in a power-off state.